DISCONTINUED OPERATIONS, SIGNIFICANT DISPOSALS AND OTHER BUSINESS EXITS - Summarized Reserve Charges (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended						9 Months Ended		12 Months Ended
	Dec. 31, 2021	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Employee termination costs
Beginning of period		$ 272	$ 764	$ 712				$ 712	$ 0	$ 0
Additional charges		9	36	225	$ 0			270	0	781
Foreign exchange		(10)	(1)	0						0
End of period		102	272	764				102		712
Severance
Employee termination costs
Beginning of period		272	731	687				687	$ 0	0
Additional charges		9	39	171						687
Foreign exchange		(10)	(1)	0						0
End of period		$ 102	$ 272	$ 731				$ 102		$ 687
Citibank Korea Inc. | Severance
Employee termination costs
Beginning of period	$ 1,052					$ 714	$ 1,054
Additional charges						(3)	31
Utilization	(1)					(670)	(347)
Foreign exchange	3					(41)	(24)
End of period	$ 1,054					$ 0	$ 714